1. Field of the Invention
The present invention relates to a satellite broadcast receiving converter, and more particularly to a Low Noise Block Down Converter (referred to as LNB hereinafter) receiving radio waves from a plurality of satellites for use in satellite broadcasting or satellite communication.
2. Description of the Background Art
At present, broadcasting satellites (three satellites referred to as DBS1, DBS2, and DBS 3) having a signal frequency band of a BSS (Broadcast Satellite Service) frequency band at 12.2–12.7 GHz are launched over the North America in the vicinity of the west longitude 101°. Two kinds of circularly polarized signals of right-hand circular polarization and left-hand circular polarization are used in the BSS frequency band as transmission signals from the satellite.
FIG. 11 is a diagram illustrating RF (Radio Frequency) signals received from a plurality of satellites.
Referring to FIG. 11, a satellite broadcasting receiving antenna receiving a satellite broadcasting signal transmitted from a satellite 500 and a satellite 502 includes a reflector plate 504 reflecting the satellite broadcasting signal transmitted from the two satellites, and an LNB 506 amplifying the satellite broadcasting signal reflected at reflector plate 504 with low noise and converting the frequency to a lower frequency band. LNB 506 is connected to a satellite broadcast receiver such as a television receiver.
A coaxial cable is normally used to receive radio waves at a satellite broadcasting antenna and to introduce the signal to an indoor BS tuner. The radio waves received at the antenna, however, cannot be introduced indoors directly with the coaxial cable.
A metal tube called a waveguide has to be used to introduce the radio waves for satellite broadcasting having an extremely high frequency. The use of the waveguide requires a big hole to be provided on a wall to introduce a signal from the antenna to the indoor satellite broadcast receiver, and causes large attenuation, and therefore it is not realistic.
Therefore, LNB 506 installed at the antenna is used to lower the frequency of the received signal to such a frequency as to be introduced even through the coaxial cable and then to transmit the signal to the indoor satellite broadcast receiver. The indoor satellite broadcast receiver includes a scramble decoder, which allows descrambling for displaying an image on a display unit.
The input portion of LNB 506 is provided with feed horns 510A and 510B corresponding to satellites 500 and 502, respectively.
Conventionally, an LNB designed for multi-satellite has been used in which the RF signals input from a plurality of satellites are switched by one LNB switch and selectively received. This LNB designed for multi-satellite receives the signal from each satellite through respective independent feed horns 510A and 510B. Therefore, the substrate having feed horns 510A and 510B connected is provided with a plurality of independent input portions. To amplify the broadcasting signal corresponding to each of the plurality of satellites that is supplied from these input portions at least one low noise amplifier circuit (LNA) is present for each satellite. In the signal processing after this LNA, a signal from a desired satellite is selected by switching.
FIG. 12 is a schematic block diagram showing a first exemplary configuration of LNB for receiving transmission signals from a plurality of satellites in the conventional satellite broadcasting receiving system.
Referring to FIG. 12, signals from satellites 500 and 502 are respectively received by feed horns 510A and 510B.
Two kinds of signals respectively received by feed horns 510A and 510B are amplified with low noise by LNA (Low Noise Amplifier) 522A and 522B.
The RF signals amplified with low noise by LNA 522A and 522B are provided to the respective one inputs of mixers 530A and 530B after unnecessary signals are removed through BPF (Band Pass Filter) 528A and 528B, respectively.
The other inputs of mixers 530A and 530B respectively receive local oscillating signals from local oscillator circuits 526A and 526B. The RF signals and the local oscillating signals are mixed in mixers 530A and 530B. As a result, the RF signals are converted to intermediate frequency (IF) signals in the intermediate frequency band.
The IF signals from mixers 530A and 530B are respectively provided to PIN diodes 532A and 532B. When PIN diodes 532A and 532B are turned on, the IF signals are passed. On the other hand, when they are turned off, the IF signals are blocked. The selection of the IF signals is performed by the individual on/off control of PIN diodes 532A and 532B.
The selected IF signal is amplified by an IF amplifier 534 and transmitted to a port PO through AC coupling using a capacitor 535. The IF output signal is output from the port PO to a satellite broadcast receiver (not shown) for example a television receiver.
FIG. 13 is a schematic block diagram showing a second exemplary configuration of the conventional LNB.
Referring to FIG. 13, an LNB 550 differs from LNB 510 in that PIN diodes 532A and 532B are not provided in the configuration of LNB 510 illustrated in FIG. 12 and mixers 530A and 530B are directly connected to IF amplifier 534.
Furthermore, it differs from LNB 510 in that a power supply circuit 540 powers a circuit portion 551 and a control portion 552 and the power supplies for LNA 522A and 522B are independently controlled by control portion 552.
The remaining configuration of LNB 550 is similar to that of LNB 510 and the description thereof will not be repeated.
LNB 550 selectively inputs a signal from the desired satellite to IF amplifier 534 by turning off either of the power supplies for LNA 522A and LNA 522B.
FIG. 14 is a diagram showing an exemplary configuration of receiving signals from a plurality of satellites where conventional LNB itself does not have a switching function.
Referring to FIG. 14, three LNB 1-LNB 3 are provided for receiving radio waves from first to third satellites. A switch box SW-BOX receives the signals received at LNB 1-LNB 3 and selects any one of them.
The selected signal is output from a port PO and input to a satellite broadcasting receiving device IRD (Integrated Receiver/Decoder) that is installed indoors.
When a plurality of LNB corresponding to a plurality of satellites are internally mounted in one package in order to realize the LNB designed for multi-satellite, the following problems arise with only the switching selection of signals that are output from the internal LNB for each satellite as in the conventional circuit configuration.
First of all, the satellite broadcast receiver that powers LNB has a limit in the current to be supplied. In order to allow a variety of conventional satellite broadcast receivers to be connected, even a satellite broadcast receiver having a small current capacity has to be handled. Although the provision of the switch box as shown in FIG. 14 allows the power to be supplied with a separate system, a compact LNB designed for multi-satellite cannot be implemented. Therefore, the power has to be supplied through a cable from the satellite broadcast receiver.
In LNB 510 shown in FIG. 12, the voltage applied from the satellite broadcast receiver through the port PO is converted into a prescribed power supply voltage by power supply circuit 540 and is supplied to circuit portion 511.
In the configuration shown in FIG. 12, however, that LNA of two LNAs which amplifies the signal from the satellite that is not received is continuously supplied with power supply current from power supply circuit 540, so that the power consumption of LNB is inevitably large.
Second, as shown in FIG. 13, if the selection operation is performed by the on/off control of the power supplies for LNA, a noise caused in the mixer portion of the unselected LNA is unpreferably combined with the received signal from the selected satellite. Furthermore, there is a demand to transmit a received signal from one LNB to a plurality of satellite broadcast receivers. In this case, such a case cannot be handled in that a signal from a first satellite is sent to a first broadcast receiver and a signal from a second satellite is sent to a second broadcast receiver, for example.